There are various types of nonvolatile memories including a flash memory, a ferroelectric random access memory, a magnetoresistive random access memory, etc. Flash memories are largely classified into a NAND type and a NOR type. The NAND type, for which the scale of integration is easily increased, is generally used for storage devices or the like such as an SSD (solid state drive) which requires a large memory capacity. The NOR type is used for storing BIOS (basic input output system), for storing logs, for storing FPGA (field programmable gate array) configuration data, etc.
In the case of an SSD employing the NAND type, the embedded SSD controller ensures reliability during operations. In contrast, the NOR type is mostly used alone. Although there is a type of memory device that is provided with an error correction function, a failure such as an inversion of logical values may occur during operations.
In flash memories, the length of write time varies greatly from memory cell to memory cell, due to variation in the size of control gates, variation in the concentration of impurities in source and drain regions, imperfection in the manufacturing of floating gates and insulating layers, etc. In consideration of this, the write voltage is adjusted on a write-unit-specific basis (i.e., on a bit-specific basis, on a word-specific basis, on a block-specific basis, or the like) during a final factory test to keep the length of write time within a specified range with respect to every memory device to be shipped.
For memory cells whose original manufacturing variation is small, no adjustment may be necessary because the length of write time lies within the specified range prior to any adjustment, or only a small-scale adjustment of the write voltage is needed to keep the length of write time within the specified range. For memory cells whose original manufacturing variation is intermediate, the write voltage may be adjusted to a fair degree to keep the length of write time within the specified range.
For memory cells whose original manufacturing variation is extremely large, it is impossible to adjust the write voltage to keep the length of write time within the specified range. Such memory cells are treated as defective cells. Addresses corresponding to the defective memory cells may be recovered by a redundancy process that utilizes backup memory cells as substitutes therefor. When recovery is not possible due to a large number of defective cells, the memory device as a whole is disposed of as a defective device.
An insulating layer, which is an element of a memory cell in a flash memory, undergoes degradation due to a high voltage applied during write operations and erase operations. Increasing degradation of an insulating layer causes the data retention property of the memory cell to deteriorate. As a result, the electric charge kept inside the floating gate may discharge with the passage of time, resulting in the retained data being lost.
Among the memory cells falling within the specified range, those memory cells existing in the write areas for which a voltage adjustment is made to increase a write voltage are subjected to a write voltage higher than an average write voltage each time a write operation is performed. As a result, such memory cells experience insulating layer degradation at faster speed than average memory cells. Especially in the write areas where the voltage is adjusted by a large amount, an applied write voltage is significantly higher than an average write voltage. Such areas thus have the problem that the data retention property is highly likely to suffer a problem in the long run.
As described above, some memory devices have memory cells for which the data retention property will suffer a problem in the long run. Even such memory devices are shipped from memory manufacturers as satisfactory products as long as the length of write time falls within the specified range for all the memory cells. The problem of data retention property in those memory devices comes to the surface during field use by end consumers, i.e., after the memory devices are embedded into products by an assembly manufacturer (i.e., the manufacturer who makes products by assembling purchased components such as memory devices for selling to end consumers). It is undesirable for the assembly manufacturer to use memory devices that are highly likely to present a problem in the long run, in products made available in the market.